Code P0300 Definition
Random/Multiple cylinder misfire detected.
What Does P0300 Mean?
P0300 indicates that two or more cylinders are experiencing misfires. A misfire occurs when an insufficient amount of fuel is burned in a cylinder. The efficient burning of fuel is essential to engine operation as the combustion of fuel is what provides the energy to power the engine. A misfire from two or more cylinders can be caused by many reasons from a faulty ignition system, fuel system, or internal engine failure. When P0300 occurs, it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine and catalytic converter(s). Many times, P0300 occurs when there are worn out spark plugs, spark plug wires, or a faulty ignition coil.
Engine Code P0300 Symptoms
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
(* = Most Common)
- Faulty or worn spark plugs *
- Faulty spark plug wires or coils *
- Distributor failure *
- Faulty fuel injector
- Vacuum leak
- Low fuel pressure
- Camshaft sensor defective
- Crankshaft sensor defective
- Engine timing off
- Leaking head gasket
- Low engine compression
- Poor quality fuel
Code P0300 Severity – Severe
P0300 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.
Code P0300 Common Diagnosis Mistakes
Loose fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.
Code P0300 Diagnosis Steps
Tools Needed to Diagnose:
Tools You May Already Have:
Tools You May Need (FIXD’s Top Picks From Amazon):
- Digital multimeter
- 5/8in. Spark Plug Socket
- Ratchet, sockets, and extensions
- Fuel pressure gauge
- Compression tester
- Leakdown tester
- Spark plugs
- Spark plug wires
How To Diagnose P0300
Difficulty of Diagnosis and Repair – 2 out of 5
- Use FIXD to scan your vehicle to verify P0300 is the only code present. If other codes are present, they must be addressed first.
- Check for loose connectors at the ignition coils or for damaged wiring. Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
- Check the condition of your spark plugs and spark plug wires. Worn and old spark plug wires are common causes of random misfires. Replace spark plugs and wires if needed and recheck for misfires. (How to identify a fouled spark plug, How to test spark plug wires, How to gap spark plugs, How to replace spark plugs)
- If you have determined that your ignition system is operating correctly, there may be a problem within your fuel system that is causing the random misfires. The following should be checked to ensure the engine is getting the proper amount of fuel.
- Check fuel pressure. Low fuel pressure can cause intermittent misfires on multiple cylinders. When the pressure is below the specification, the engine does not receive the proper amount of fuel and will start to lean misfire. The fuel pump or fuel pressure regulator could be the source of the low fuel pressure. (How to check fuel pressure)
- Check that the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also, check that the fuel injector wiring is not damaged and is connected properly. (How to check fuel injectors by ear, How to check fuel injectors with a digital multimeter)
- If the ignition system and fuel system checks out, you may want to perform an engine compression test and leakdown test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
- Leaking Head Gasket
- Broken valve spring
- Broken piston ring
- Worn valve guides
- Burned valve
- Timing chain or belt skipped tooth and engine is off time.
Estimated Cost of Repair
For error code P0300, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Spark plugs $66-$250 (depending on if you take it to the shop or do it yourself)
- Ignition Coils $230-$640 (some cars require Intake manifold removal)
- Spark plug wires $180-$240
- Fuel injectors $1500-$1900
- Vacuum leak $100-$200
- Fuel pump $1300-$1700
- Fuel pressure regulator $200-$400